National Hurricane Center Director Max Mayfield said Saturday afternoon that Hurricane Katrina will be at least a Category 4, with winds of 145 mph when it approaches the New Orleans area, and that it could be a Category 5, with winds of 155 mph or higher.

Meanwhile, computer model runs conducted by a team of Louisiana State University scientists indicate that even if Katrina had winds of only 115 mph, levees protecting Kenner, Metairie and New Orleans on the east bank will be overtopped by a 10- to 12-foot storm surge, topped by waves at least half that high, in some locations along Lake Pontchartrain.

"The guidance we get and common sense and experience suggests this storm is not done strengthening, " Mayfield said in a telephone call from Miami-Dade County, which was hit by a Category 1 Katrina earlier in the week.

"This is really scary, " he said. "This is not a test, as your governor said earlier today. This is the real thing."

Mayfield warned residents intent on not leaving in advance of Katrina to learn from the storm's effects in south Florida.

"We think this was a solid Category 1 when it made landfall here, " he said. "But we had windows in new homes that blew in when they were designed for 145 mph winds."

Mayfield also warned people not to focus on the eye of the storm, as atmospheric conditions are perfect for Katrina's intensity to stretch far to the east and west of its eye.

"This thing is like Hurricane Opal, " he said, referring to a huge 1995 hurricane that hit the Florida panhandle as a Category 3 storm. "We're seeing 12-foot seas along the Louisiana coast already."

Ivor van Heerden, a scientist at the LSU Hurricane Center, said he's also concerned that the waves atop the surge in Lake Pontchartrain could weaken levees and cause additional overtopping.

"The bottom line is this is a worst-case scenario and everybody needs to recognize it, " he said. "You can always rebuild your house, but you can never regain a life. And there's no point risking your life and the lives of your children."